1. Field of the Invention
The present invention relates to a thermal transfer sheet, and particularly relates to a thermal transfer sheet comprising a heat resistant layer and a slip layer provided on its back surface, which is excellent at response to a heated thermal head such as slipping property or releasability, capable of preventing formation of a head grime in a heating operation by the thermal head and a cooling process thereafter, and also preventing crumples of the thermal transfer sheet or an image-receiving material in a printing process, thereby providing an printed image with high quality. The present invention further relates a method for manufacturing such a thermal transfer sheet.
2. Description of the Related Art
As conventional thermal transfer sheets, there have been known a sublimation type thermal transfer sheet and a heat fusion type thermal transfer sheet. A typical sublimation thermal transfer sheet is composed of a substrate film made of plastic such as polyester and a dye layer as a thermally transferable coloring material layer which is disposed on one surface of the substrate film and made of sublimation dye and binder resin. On the other hand, the heat fusion thermal transfer sheet has a heat fusible ink layer made of a heat fusible composition containing coloring material instead of the dye layer. Such a thermal transfer sheet is image-wise heated from its back surface side by a heating means such as the thermal head to transfer the dye of the dye layer or the heat fusible composition of the heat fusible ink layer to the image-receiving material, thus forming an image.
Use of the conventional thermal transfer sheet having a substrate film made of relatively heat fusible material such as plastic has caused problem in a process for formation of the image, such as deterioration of the releasability and the slipping property to the thermal head and breakage of the substrate film. In order to solve that problem, a heat resistant ship layer has been formed on a surface of the substrate film opposite to the surface on which the coloring material layer is disposed by using modified resin such as thermosetting resin and silicone resin solely or in combination with cross linking agent. However, along with improvements in printing speed and printing quality of a printer, there has been desired a more excellent heat resistance and slipping property of the heat resistant slip layer, and has also been desired to reduce estrangement of the slipping property effected at a printing time from that demonstrated in an non-printing time. Accordingly, there has been made an attempt to add a lubricant such as surface active agent, oil, organometallic salt and wax any one of which has the good slipping property and the good releasability in a heated condition into the heat resistant slip layer.
At the time of image formation, printing energy to be applied to the thermal transfer sheet by the thermal head is varied in a wide range according to respective printing densities, and the slipping property and the releasability are desired to be stable within the whole range of the printing energy. However, the conventional lubricant has still caused problem.
More specifically, in a case where the lubricant to be added is liquid, it may have a poor compatibility to the binder resin for the heat resistant slip layer. Furthermore, the liquid lubricant may transfer to undesirable place. Firstly, when the liquid lubricant has a low viscosity, the lubricant may transfer to the opposite surface of the substrate film or a surface of a conveying roll in the manufacturing or working process to cause a shortage of the lubricant in the heat resistant slip layer of the thermal transfer sheet as an end product. Accordingly, the use of the liquid lubricant may cause a deterioration of the slipping property. Second, when the thermal transfer sheet is rolled up, the use of the liquid lubricant may cause the transferring of the coloring material from the coloring material layer to the adjacent heat resistant slip layer, resulting in contamination of the heat resistant slip layer. Third, when an intermediate product of the thermal transfer sheet which has the heat resistant slip layer but no coloring material layer is rolled up, the liquid lubricant may transfer from the heat resistant slip layer to an exposed front surface of the adjacent substrate film, thereby causing deterioration of wettability of the substrate film to a coating solution for the coloring material layer.
On the other hand, in another case where the lubricant to be added is solid or wax, the slipping property and the releasability may be insufficient because of its slow response to a momentary heating, and further, the lubricant may be deposited on the surface of the thermal element such as the thermal head to become the head grime in a cooling process after the heating with the use of the thermal head, thus causing a bad influence to the printed surface. As shown in FIG. 2, when the thermal head 8 is sliding along the back surface 7 of a conventional thermal transfer sheet 101, the head grime 10 is liable to be deposited on a surface of an advance direction (9) side of the thermal head 8.
In another problem of the conventional thermal transfer sheet, when each printing line of the thermal transfer sheet which is a portion to come contact simultaneously with the heating means such as the thermal head includes printed portions to be heated by the heating means and non printed portion not to be heated, the slipping property of the heat resistant slip layer to the heating means, i.e. a friction coefficient, varies in accordance with heat energy applied from the heating means, and may be involved in un-uniform in dependence on the portions to cause crumples of the thermal transfer sheet, resulting in the dropping out of print.